388 THE Cx4.NAI)lAN NATURALIST. [Yol. TU. 



to one another aud appear under diverse forms, whether they are 

 passing from atoms to ether or from ether to atoms : but we 

 never see them disappear or lose their force— only transfx^rra 

 themselves and perpetually renew their youth. 



And this is not all. These vibratory movements which sway 

 atoms and which whirl about in ether can cause movements ci' 

 the mass, displacement either of the bodies or of the molecules. 

 Warm a bar of iron, it will dilate with a force almost irresistible ; 

 a part of the heat will be employed in producing a certain pull- 

 asunder of the molecules. AVarm a gas, it will in like way dilate, 

 and a part of the heat disappearing as such, will produce a sepa- 

 ration very considejablo in this case between the gaseous mole- 

 cules ; and the proof of the consumption of heat in work of dila- 

 tion is not difficult to give, for if you warm the same gas to the 

 same degree, but prevent it from dilating, less heat need be given 

 to it than in the former case. The diflferance between the two 

 quantities of heat corresponds exactly to the mechanical work 

 performed by the molecules in dilatation. That is one of the 

 most simple considerations, on which is founded the principle 

 of the mechanical equivalent of heat so often now referred to in 

 mechanics, in physics, and in physiology. 



In physics it explains the mystery of latent heat, of fusion, 

 and of volatilisation. But how is it that heat supplied con- 

 tinuously to a boiling liquid to maintain ebullition does not ever 

 raise the temperature of the liquid above a point which under 

 similar pressure remains fixed ? The reason is that this heat is 

 continually absorbed, and diappears as such to produce the 

 mechanical work of driving apart the molecules. And so in the 

 phenomena of fusion, the constancy of the temperature indicates 

 the absorption of the heat consumed in molecular work. These 

 conceptions have modified and thrown ma^h light on the defini- 

 tions which physicists have applied to diie states of matter, 

 and it is seen that they are in harmony with chemical theories 

 of the constitution of bodies. These are formed of molecules 

 which represent systems of atoms animated by harmonic move- 

 ments, and whose equilibrium is exactly maintained and strength- 

 ened by these movements. 



Applied to molecules thus constituted, heat can produce three 

 dijfferent effects. In the first place, an elevation of temperature 

 by the increase of vibratory energy ; in the second place, an increase 

 of volume by the driving tipart of atoms and molecules, and this 



